This research will focus on the neurophysiological mechanisms of sound localization in the owl. Neurons in one part of the owl's midbrain auditory nucleus respond only to sound coming from sharply defined areas in space. Their receptive fields are organized so as to generate a neural map of auditory space. The map assigns predetermined spatial coordinates to each binaural input. Space and frequency are represented separately in the nucleus, suggesting functional divisions within the auditory system. It is proposed here to study the nature and causal mechanisms or receptive fields and the map with a combination of dichotic and free-field stimulus presentation. Histological techniques will be used to discover the neuroanatomical correlates of the function division within the auditory system. The map predicts that the accuracy of sound localizations will vary according to the aximuthal position of the target. This will be tested behaviorally. The position of a unit's receptive field depends on a delicate balance of sound intensity at the ears. The effects of monaural occlusion during ontogeny will be assayed both neurophysiologically and behaviorally in search of developmental plasticity in the auditory system.